Gas-measuring devices are used to detect and to monitor gases and vapors in the ambient air of a human user. Gas-measuring devices are important, in particular, for detecting and monitoring toxic gases in an industrial environment and at the workplace. A very large number of toxic substances, which jeopardize the health of persons who are present, may occur in the gas phase of the ambient air in such fields of application.
Therefore, there are limit value concentrations specified by law, which must not be exceeded (maximum allowable concentrations [at the workplace], MAC from TRGS 900) for such substances. Gas-measuring devices that are portable and can be carried by the user are called personal air monitors (PAM). Strict requirements are imposed on such PAM gas-measuring devices in terms of the quantifiability of the gas measurement, reliability, safety, operability and especially the measuring time (and hence the rapidity with which a warning is generated).
The sensor or sensors, which are preferably based on chemical principles, represents/represent an important component of any gas-measuring device. Each sensor comprises at least the receptor and the transducer. The receptor interacts with analyte molecules (i.e., the gas molecules to be detected in the substance, which is, for example, a toxic substance) at the molecular level. The physicochemical property of the receptor changes in the process. This change is detected by the transducer and converted into an electrical signal.
FIG. 1 shows, for example, a gas sensor signal, which is sent when the analyte concentration in the area of the receptor assumes a rectangular course over time. The time is plotted on the x axis and the intensity I of the sensor signal S and of the analyte concentration K on the y axis. If an analyte concentration K changing in this manner over time is admitted to the receptor, the sensor usually responds with a rapid rise (response) of the transducer signal S up to a maximum, which essentially corresponds to the analyte concentration K. This change in the signal takes place during the measurement phase. The response time of a sensor is by definition (EN45544-1: 1999) 90% or 50% of the maximum signal intensity (t90, t50). The shorter the response time, the sooner can the warning about the analyte take place.
If the feed of analyte is stopped, the transducer signal usually also tends towards zero. This change in the signal takes place during the regeneration phase. The recovery time (recovery), which is defined at a drop to 10% of the signal maximum, is used in this case for the characterization.
The overall size, weight and energy consumption play an important role especially in personal air monitors. This correspondingly also applies to the gas sensors used in the gas-measuring devices.
Compact sensors with low power consumption are, for example, the capacitively-controlled field effect transistor (CCFET) gas sensors based on the mySENS technology of Micronas, Freiburg, which are also explained in the article by H. P. Frerichs, I. Freund, K. Hoffmann, T. Kolleth, C. Schladebach, C. Wilbertz: “Platform of Cost-Effective Gas Sensors Based on the CMOS Technology,” Conference Proceedings: Sensoren im Automobilbau [Sensors in Automotive Engineering].
U.S. Pat. No. 3,906,473 describes a semiconductor sensor for the detection of carbon monoxide, which responds sensitively to CO at low sensor temperatures. The different reactivities of carbon monoxide and hydrocarbons at different sensor temperatures are used in U.S. Pat. No. 4,012,692 to distinguish the analytes. A semiconductor-based sensor is likewise operated at different temperatures in U.S. Pat. No. 4,185,491. Modifications are described in the documents U.S. Pat. No. 4,399,684, U.S. Pat. No. 4,567,475 and EP 0 092 068. A method for operating a breathing gas analyzer based on field effect transistor-based sensors, which provides for a measuring operation and a regeneration phase lasting up to one hour, is described in WO 2012/100979A1. DE 199 26 747 describes a receptor for detecting ammonia.
The article “H2, CO and high vacuum regeneration of ozone poisoned pseudo-Schottky Pd—InP based gas sensor” by L. Mazet, C. Varenne, A. Pauly, J. Brunet, J. P. Germain, published as “Sensors and Actuators B 103 (2004) 190-199” Elsevier, described different response behaviors and desorption methods of pseudo-Schottky based gas sensors.